Social neuroscience, devoted to studying how biological systems implement social processes and behavior, has revealed crucial insights into the inherently social nature of humans. Most of the so far available findings, however, are derived from investigations examining biological processes and brain activation patterns in participants who were presented with social stimuli while being alone. Because “social cognition is fundamentally different when we are in interaction with others rather than merely observing them” (Schilbach, 2013), there currently is a strong motivation to implement truly interactive “second-person” social neuroscience paradigms to overcome this limitation.

One promising way to observe two individuals directly interacting with each other using state-of-the-art social neuroscience techniques is to apply functional near-infrared spectroscopy (fNIRS). fNIRS measures blood-oxygen-level dependent (BOLD) signal change as an indirect correlate of brain activity with the help of infrared light absorption. Although fNIRS can only reach the cortical surface, it offers many advantages over other neuroimaging techniques. Among others, it is minimally invasive, relatively motion insensitive, has a reasonably good spatial and temporal resolution, and allows for much more ecologically valid experimental setups. Furthermore, besides measuring change in brain activity in each participant separately, fNIRS hyperscanning – combined with behavioral ratings of videos recorded during interaction – allows to derive measures of bio-behavioral synchrony. In other words, fNIRS can be used to determine whether brain activity during social interaction shows a similar pattern in the same brain area(s) in both participants at the same time, and whether such increase in inter-brain coherence is associated with behavioral attunement and/or better behavioral task performance.

But why are inter-brain coherence and bio-behavioral synchrony during social interaction of interest? The general idea is that for successful social interaction, the oscillatory processes in the interacting individuals’ brains have to become synchronized to each other so that information of any sort can flow between them – “analogous to a wireless communication system in which two brains are coupled via the transmission of a physical signal (light, sound, pressure or chemical compound) through the shared physical environment” (Hasson et al., 2012). Within this context, interpersonal synchrony is not only considered important at the neural (i.e. inter-brain coherence), but also at the behavioral (e.g. eye gaze, touch), physiological (e.g. heart rate), and endocrine (e.g. cortisol or oxytocin secretion) levels (Feldman, 2017). And this is where attachment comes into play. Social synchrony is learned within the parent-infant bond, which means that attachment contexts provide the arena for the experience and encoding of synchrony early in life. Furthermore, it is thought that social synchrony experienced during early sensitive periods provides the foundation for the expression of synchrony in later attachment bonds throughout the life span (Kinreich et al., 2017). For the emergence of attachment, however, it appears that not only social synchrony is of great importance. What also seems to be crucial are experiences of asynchrony as deviations from allostasis (and a subsequent return to synchrony and thus allostasis) – e.g. when a child becomes upset and the parent tries to calm her/him down. These sequences of synchrony, asynchrony, and re-synchrony provide ideal opportunities for the child to learn about the social environment, allowing him/her to start building predictions about the own capacity to elicit care and the responsiveness of others to help co-regulating emotions when needed (Atzil et al., 2018).

We are currently running a series of investigations along the above lines, during which we assess bio-behavioral synchrony in parent-child dyads (using fNIRS and behavioral rating of interaction videos) involving both mothers and fathers together with their children (CARE Studies). The results from an earlier study conducted at Stanford University in 28 mother-child pairs (child age 8-12 years) have just been accepted for publication (see here and publications page).

In this study, we show that inter-brain coherence in mother-child pairs generally increases across a set of four right prefrontal and one right lateral temporal cortical brain area(s) during a collaborative versus an independent visual reaction time task (see Figure 1, panels A and B) – particularly so in the dorsolateral and frontopolar prefrontal cortex (see Figure 1, panels A and C). Moreover, we report an overall sex-difference in this pattern, because differential inter-brain coherence was only present in mother-son but not mother-daughter dyads (see Figure 1, panel D). Our study is among very few to date reporting inter-brain coherence during mother-child interaction, and the first to show sex-differences in these patterns. Due to the preliminary nature of our findings, however, it is difficult to provide a concluding interpretation, particularly regarding the observed sex-differences. Although sex-differences in inter-brain coherence during cooperation have been found before in adult-adult dyads (Baker et al., 2016; Cheng et al., 2015), the so far available results do not converge yet and may reflect different underlying neural processes as compared to in parent-child dyads. More research is needed to replicate and further extend the parent-child data, for example by also including father-child interactions.

What we also tried for the first time in the above study is to associate bio-behavioral synchrony, and particularly inter-brain coherence during mother-child interaction, with a measure of parent-child attachment. To do so, we obtained child attachment towards the mother using a validated child version of the Experiences in Close Relationships self-report questionnaire in its revised version (ECR-R; ECR-RC). Associations between inter-brain coherence and child attachment towards the mother in this study, however, remain inconclusive. Although there was weaker inter-brain coherence increase during collaboration (versus the independent condition) in dyads within which children reported having a more avoidant attachment style towards their mothers in one area of the right frontopolar prefrontal cortex (p= .038; see Figure 2, panels A-C), this association did not survive correction for multiple comparisons and seemed to also partially depend on mother age and/or child gender. More research is therefore clearly needed to further extend our knowledge on inter-brain coherence and its association with attachment in the mother- and also father-child interaction.

We are aiming at clarifying several of the above-mentioned issues by performing fNIRS hyperscanning in larger samples of both mother-child and father-child pairs (child age 5 years, both girls and boys), and by obtaining attachment using several different procedures in both parents and children within our CARE studies. We hope that data from these additional studies will provide more information about the biological and brain basis of social interaction in general, and parent-child interaction in particular, and its potential links to parent-child relationship quality & attachment.